Our goal is to get as many people signed up as possible, because general retention rates for ExCo classes are around 15% by the end of the semester. (Last semester, 70 people signed up, 8 people tested for 6th kyu at the end of the semester)

Perhaps the issue here is, then, not that you need more people joining at the beginning of the semester but to take a look at the curriculum to see if what is being taught during the semester and how it is being taught should be examined?

In any case, I wonder what sorts of changes people here have had to make in their beginners curriculum in order to try to help their retention rate.